1. Field of the Invention
The present invention relates to touch panels for display devices. More particularly, the present invention relates to combination capacitive-type and electromagnetic type touch panels for display devices.
2. Discussion of the Related Art
Touch panels have been developed as a means of efficiently interfacing with electronic devices via a display surface. For example, users may input desired information using a touch panel integrated with a display device while watching images displayed by the display device. Allowing users to input desired information to an electronic device via a display surface, touch panels substantially reduce or eliminate the need for other types of input devices (e.g., keyboards, mice, remote controllers, and the like). Currently, touch panels have been widely integrated with display surfaces of flat panel display devices such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, electroluminescence (EL) devices, and cathode ray tubes (CRTs).
Depending on the type of contact object used (e.g., a user's finger, a stylus, etc.), and depending on the manner in which the location of a contact point (i.e., the location where the contact object is operably proximate the touch panel) is determined, touch panels are generally classifiable as analog resistive-type, capacitive-type, electromagnetic (EM)-type, saw-type, and infrared-type touch panels.
Generally, analog resistive-type touch panels include an upper transparent substrate supporting an upper electrode and a lower transparent substrate supporting a lower electrode. The upper and lower transparent substrates are attached to each other but spaced apart from each other by a predetermined distance. When a surface of the upper transparent substrate is contacted by a contact object, an upper electrode formed on the upper transparent substrate electrically contacts a lower electrode formed on the lower transparent substrate. When the upper and lower electrodes electrically contact each other, a voltage, made variable by a resistance value or a capacitance value specific to the location of where the contact object contacted the touch panel (i.e., the contact point), is then detected and outputted along with a location defined by coordinates of the contact point.
Generally, capacitive-type touch panels include a transparent electrode formed on a display device such as an LCD panel, wherein a voltage is applied to each corner or side of the transparent electrode and a uniform electric field is thereby generated within the transparent electrode. Coordinates of the contact point are determined in accordance with a voltage drop generated when the user touches the touch panel via the contact object.
FIG. 1 illustrates a schematic view of a related art electromagnetic (EM)-type touch panel for a display device.
Electromagnetic (EM)-type touch panels generally determine coordinates of a contact point by detecting an electromagnetic field resonance. Recently, related art EM-type touch panels have been integrated with Tablet PCs, capable of displaying high-quality images.
Referring to FIG. 1, a related art EM-type touch panel is integrated with a display device such as an LCD device, wherein the LCD device includes an LCD panel 11, an upper polarizing plate 12, a lower polarizing plate 13, a backlight 14, a passivation layer 15, an EM-type touch panel, and a case top 17.
The LCD panel 11 is capable of displaying images in accordance with externally input driving and video signals and includes upper and lower substrates bonded to each other and spaced apart from each other by a predetermined distance, wherein liquid crystal material is injected between the upper and lower substrates. The upper polarizing plate 12 is arranged over the LCD panel 11 and the lower polarizing plate 13 is arranged beneath the LCD panel 11 to selectively polarize light irradiated by the backlight 14 into the LCD panel 11 as well as emitted light transmitted by the LCD panel 11. Arranged above and spaced apart from the upper polarizing plate 12, the passivation layer 15 serves as a dielectric layer as well as protects the LCD panel 11 from a proximately arranged stylus pen 23, as will be discussed in greater detail below. The EM-type touch panel 16 is arranged below the LCD panel 11 and outputs a variable voltage in accordance with a location of the contact point on the touch panel. The case top 17 is provided as a metal material that secures the backlight 14, the LCD panel 11, and the EM-type touch panel 16 together as a single body.
The EM-type touch panel 16 includes a sensor board 22 having a sensor PCB 18, a shield plate 19, and a connector 21 to generate electromagnetic fields; and a control board 25 having a microprocessor 24 for transmitting signals to the sensor board 22 and for detecting coordinates of contact point generated by a stylus pen 23 by detecting input signals generated by the stylus pen 23, wherein the stylus pen 23 includes a resonance circuit 26 having a coil and a capacitor. The electromagnetic field generated from the sensor board 22 is stored in the resonance circuit 26 of the stylus pen 23 for a predetermined amount of time.
During operation of the related art EM-type touch panel shown in FIG. 1, a control signal is generated by the control board 25 which, in turn, enables the sensor board 22 to generate an electromagnetic field. Subsequently, a current is induced within a stylus pen 23 arranged within the generated electromagnetic field, wherein the induced current has a resonant frequency and is stored for a predetermined amount of time in accordance with an LC value of the resonance circuit 26. The sensor board 22 then detects the induced current stored within the resonance circuit 26 and transmits a corresponding signal to the control board 25 whereby the control board 25 determines the location of the contact point generated by the stylus pen 23.
The resonance circuit 26 is provided as an LRC circuit, wherein the amplitude of the induced current varies with the frequency of an applied power source. The amplitude of the induced current is maximized at a predetermined resonance frequency (f) of the applied power source. More specifically, the resonance frequency (f) is determined by the following equation:f=(2π√{square root over (LC)})−1 (L is an inductance value of an inductor, and C is a capacitance value of a capacitor).
As can be seen from the above discussion, related art EM-type touch panels determine the contact point of a stylus pen using the resonance frequency of transmitted electromagnetic fields, thereby detecting contact points according to a completely different method capacitive-type touch panels use to detect contact points. Further, contact objects, such as a user's finger, do not affect the operation of related art EM-type touch panels. Because the stylus pen is arranged within the generated electromagnetic field, the location of the stylus pen relative to the EM-type touch panel can be detected even when it is above the passivation layer so that hovering and variable pressure effects can differentiated. EM-type touch panels, therefore, are commonly designed for use in settings such as conferences, seminars, etc., to eliminate the number of potential contact objects from disturbing the EM-type touch panel. The related art EM-type touch panel, however, has a complex circuit structure and is difficult to integrate with existing liquid crystal display modules (LCMs). Moreover, related art EM-type touch panels respond only to corresponding stylus pens, thereby making it difficult apply EM-type touch panels for use in certain applications (e.g., industrial fields). Lastly, if a contact object such as a user's finger contacts the related art EM-type touch panel, no contact points are detected. Accordingly, if a stylus pen becomes lost or damaged, the EM-type touch panel becomes inoperable until a new stylus pen is provided.